1. Field of the Invention
The present invention relates to semi-transmission type liquid crystal display device having a reflection portion and a transmission portion together in one pixel region.
2. Description of the Related Art
In recent years, mobile phones, personal data assistants (PDAs), and other portable information terminals have spread in use. Demand for liquid crystal display devices, which have the advantages of thin shape and the light weight, is rising as a result.
Transmission type liquid crystal display devices for display utilizing backlight consume large power since the backlight is always turned on. Further, under sunlight or other very strong ambient light, the ambient light becomes stronger than the display brightness, so the viewability is remarkably deteriorated (“washout”). For these reasons, these are not suitable as the display devices of portable information terminals for which a reduction of the power consumption is demanded and for which a good viewability under any environment is demanded.
On the other hand, reflection type liquid crystal display devices for display utilizing external light consume very low power since backlight is not required and in principle do not suffer from washout even under sunlight. For this reason, they are suitable as the display devices of portable information terminals. In dark places where the ambient light is weak, the display becomes hard to see, but it is possible to arrange a front light at the display surface side of a reflection type liquid crystal display device and turn on the front light to secure the viewability under all environments.
As a reflection type liquid crystal display device, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-154817 discloses a normally black mode using one phase difference plate on the display surface side.
Further, Japanese Examined Patent Publication No. 3236504 discloses a normally white mode using two phase difference plates on the viewing surface side.
In the front light system, the front light is arranged at the viewing surface side, so there are the defects that a reduction of the contrast is induced and the viewability is lowered. As a method of avoiding this, a semi-transmission type liquid crystal display device provided with a reflection portion and a transmission portion in each pixel portion and arranging a backlight at the back surface is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-333624. In this method, since nothing is arranged at the viewing surface side, the contrast is not lowered and the viewability is good under all environments.
Generally, semi-transmission type liquid crystal display devices as well come in the normally white mode and the normally black mode.
FIG. 1 is a sectional view of a semi-transmission type liquid crystal display device of the normally white mode.
In this semi-transmission type liquid crystal display device, at the back surface side, a first surface side of a transparent substrate 1a has a phase difference plate (λ/4) 2a, a phase difference plate (λ/2) 3a, and a polarization plate 4a stacked on it. A transparent electrode (ITO film) 5 is formed in a region of approximately half of a second surface side of the transparent substrate 1a, an insulator 6 thicker than the transparent electrode 5 is formed in the region where the transparent electrode 5 is not formed, and a reflection electrode 7 is formed on the insulator 6. On the viewing surface side, at the back surface side, a first surface side of a transparent substrate 1b has a phase difference plate (λ/4) 2b, a phase difference plate (λ/2) 3b, and a polarization plate 4b stacked on it. A common electrode 8 is formed on a second surface (upper surface) side of the transparent substrate 1b. Further, the transparent electrode 5 and reflection electrode 7 and the common electrode 8 are bonded so that they face each other. Liquid crystals 9 are sealed between these electrodes.
FIG. 2 is a sectional view of a semi-transmission type liquid crystal display device of a normally black mode (for example refer to IDW′00 Preprints LCT2-2, p. 41–44). In FIG. 2, for easy comparison of the configuration with the semi-transmission type liquid crystal display device of the normally white mode of FIG. 1, the same components are given the same reference numerals. The points of difference of the semi-transmission type liquid crystal display device of the normally black mode from the semi-transmission type liquid crystal display device of the normally white mode are that only one phase difference plate (λ/2) 3b is arranged on the viewing surface side and that no phase difference plate is arranged on the back surface side.
As will be understood also from FIG. 1, the semi-transmission type liquid crystal display device of the normally white mode uses two phase difference plates on the viewing surface side and two phase difference plates on the back surface side, i.e., a total of four phase difference plates. This becomes a cause of higher cost. Further, in recent years, advances have been made in reducing the thickness of liquid crystal display devices, but if using these four phase difference plates, reduction of thickness inevitably becomes difficult.
On the other hand, the semi-transmission type liquid crystal display device of the normally black mode, as shown in FIG. 2, uses one phase difference plate on the viewing surface side, but does not use a phase difference plate on the back surface side, so enables realization of a lower cost in comparison with the normally white mode and is further advantageous in the reduction of thickness. Note that the orientation mode of the liquid crystal disclosed here is the homogeneous orientation.
Summarizing the problems to be solved by the invention, based on IDW′00 Preprints LCT2-2, p. 41–44, red (R), green (G), and blue (B) color filters were formed in the substrate 1b on the viewing surface side of the semi-transmission type liquid crystal display device of the normally black mode system and the chromaticities of colors in the reflection mode and the transmission mode in the case where voltages were supplied to the pixels of the different colors were measured. In the same way, a semi-transmission type liquid crystal display device of the normally white mode system was prepared and the chromaticities measured.
FIG. 3A is a view of results of measurement of reflection chromaticities of semi-transmission type liquid crystal display devices of a normally white mode and a homogeneously oriented normally black mode. FIG. 3B is a view of results of measurement of the transmission chromaticities of semi-transmission type liquid crystal display devices of a normally white mode and a homogeneously oriented normally black mode. In FIGS. 3A and 3B, the characteristics indicated by NW show the results of measurement of the normally white mode, and the characteristics indicated by HNB show the results of measurement of the homogeneously oriented normally black mode.
As will be understood from the figures, a homogeneously oriented normally black mode semi-transmission type liquid crystal display device has the disadvantage that a remarkable reduction of the chromaticity is seen in comparison with the chromaticity of the normally white mode semi-transmission type liquid crystal display device, so it is very poor in image quality.
Further, in the case of a homogeneously oriented normally black semi-transmission type liquid crystal display device, in order to realize a good dark state in the reflection portion, when the thickness of the liquid crystal layer of the reflection portion is “d”, Δnd=135 nm, and the phase difference value of the phase difference plate on the viewing surface side is preferably 275 nm. In terms of production, the refractive index difference Δn of a liquid crystal material for which reliability can be secured is 0.07 or more. When further considering the response of the liquid crystals and other characteristics, 0.075 or more is more preferred When using a liquid crystal material of Δn=0.075, the gap of the liquid crystal layer of the reflection portion becomes 1.8 μm. Usually, when lower than 2 μm, short-circuits of the upper and lower substrates and gap failure due to entry of foreign matter frequently occur. For this reason, a normally black mode semi-transmission type liquid crystal display device using homogeneous orientation has the disadvantage that production is very difficult.
Further, when using homogeneous orientation, there are the disadvantages that the inclination of a transmittance curve with respect to the voltage is gentle, a reduction of the voltage is very difficult, and, as a result, a reduction of the power consumption is difficult.
Further, in this homogeneously oriented normally black mode semi-transmission type liquid crystal display device, a phase difference plate is not used on the back surface side, only a polarization plate is used. In this case, wavelength dispersion cannot be compensated for in the transmission light. Therefore, the dark state, that is, the black color, becomes colored. In actuality, in a homogeneously oriented semi-transmission type liquid crystal display device, it was confirmed that the dark state was colored blue, so the image quality was not good.
Further, in a normally black mode semi-transmission type liquid crystal display device, when the shape of the boundary between the transmission portion and the reflection portion is rectangular, there is the disadvantage that light is reflected at the reflection electrode portion (refer to FIG. 2) of the recessed portion of the reflection portion step difference, so the reflection contrast is lowered.